A transistor is an electronic device that controls voltage and current flow. In order to mitigate the effects of impairments such as noise, transistors may be arranged so that they form a differential amplifier. A differential amplifier may be designed such that its outputs are proportional to a difference between two inputs to the differential amplifier, and, therefore, any error that may be common to both inputs may be ignored. These errors may include internal and/or external errors. Internal errors due to temperature changes may affect voltage and current characteristics of the transistors on both sides of the differential amplifier to approximately the same degree. This may occur in instances where the transistors are on the same chip and may have the same process variations.
External errors may include noise picked up by inputs of the differential amplifier. Since both signals to the two inputs of the differential amplifier may pick up the same noise, the noise may not affect the output. This may occur because the same noise on both inputs may not affect the difference between the two inputs. The noise common to both inputs may be referred to as common mode noise. In this regard, the differential amplifier may be adapted to eliminate common mode noise.
Differential amplifiers may form the basis of operational amplifiers, which may also be referred to as op amps. An operational amplifier is basically a differential amplifier having a large voltage gain, very high input impedance and low output impedance. The operational amplifier has an “inverting” or negative (−) input and “non-inverting” or positive (+) input, and one or two outputs. The high input impedance allows minimal current in to the inputs of the operational amplifier. This feature may be used with a feedback circuit, in which an output of the operational amplifier circuit depends for the most part on the feedback circuit. Often, analysis of a circuit utilizing an operational amplifier may assume the operational amplifiers to be ideal operational amplifiers. An ideal operational amplifier has infinite input impedance, zero output impedance, infinite open loop gain when there is no feedback, and infinite bandwidth.
Some operational amplifiers, such as type 741 by Fairchild Semiconductor, may have very high open loop gain on the order of several hundred thousand, but very low open loop bandwidth of 10 hertz or less. With the addition of feedback devices, for example, resistors, the closed loop bandwidth may be increased at the sacrifice of the closed loop gain. Generally, the gain-bandwidth product for an operational amplifier, which may be the gain multiplied by the bandwidth for a specific feedback configuration, may be a constant. Operational amplifiers may have a single output or differential outputs. Differential-output operational amplifiers may have outputs whose voltage levels are centered about a common mode voltage, which may be a reference voltage, and the average of the output voltages may be the common mode voltage.
Common mode noise may affect the differential outputs so that the common mode voltage may move to a level other than the desired reference voltage. Therefore, in order to reduce the effect of the common mode noise and to keep the common mode voltage at the desired reference voltage, a feedback circuit may be implemented as part of the differential-output operational amplifier. Some feedback circuit may utilize a voltage comparator and resistors. However, the drawback to this may be additional power and layout area needed.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.